1. Field of the Invention
The present invention relates to an apparatus for measuring the speed of a moving land vehicle, and more particularly to an apparatus and a method for measuring the speed of a moving land vehicle using an accelerometer.
2. Description of the Related Art
Generally, vehicles (such as ships, aircrafts and cars) have an embedded navigation system that tracks the position of a vehicle, routes a path to a given destination and provides the routing results. In order to provide a path to the destination, navigation systems should be able to determine the exact position of a vehicle.
Therefore, navigation systems generally include a positioning device for position determination. Such positioning devices are classified into two types, one for determining the position using an outside source and the other for determining the position using an inside sensor. A GPS (Global Positioning System) is an example of the former type of positioning device. A DR (Dead Reckoning) system using an inertial sensor is an example of the latter type of positioning device.
A DR system comprising an inertial sensor, a so-called INS (Inertial Navigation System), was developed for the first time by the Massachusetts Institute of Technology in the U.S. at the beginning of the 1950s and put to practical use in the 1960s. INS systems calculate the speed and position of a moving vehicle using a gyroscope for detecting a rotational motion and an accelerometer for detecting a straight line motion.
The basic principle of operation of the INS systems is summarized as follows. INS systems autonomously calculate the current speed and position of a vehicle by integrating an output from the gyroscope that measures a rotational angular speed to obtain a moving direction angle of the vehicle, compensating for a gravitational acceleration from an output from the accelerometer and then integrating the resulting values. INS systems can provide accurate and continuous navigation data during a short period of time. However, errors may accumulate with the lapse of time due to the integration processes. In order to practically use the INS system, expensive and precise gyroscopes and accelerometers are required. Most INS systems are used together with a non-inertial auxiliary sensor, such as a magnetic compass or a GPS, rather than being used alone, to ensure high accuracy and long-term stability.
As explained above, the speed of a moving vehicle can be calculated by combining speed information obtained from an accelerometer with moving direction information obtained from a gyroscope.
Generally, in the known systems, three mutually orthogonal uniaxial gyroscopes and three mutually orthogonal uniaxial accelerometers are required to calculate the accurate speed of a vehicle in three-dimensional space. For certain kinds of vehicles, the speed can be obtained using a lesser number of sensors. In the case of a car, for example, the roll motion which is the rotational motion of the wheel axles and the straight line motion in a direction perpendicular to the ground surface can be ignored when calculating the speed of the car. Since sensors for detecting such motions are not required, the number of sensors for calculating the speed of a car can be reduced by the number of such unnecessary sensors.
In order to obtain a velocity vector of a vehicle moving on the road using a DR system with an inertial sensor, it is necessary to measure the moving direction angle of the vehicle and the speed in the moving direction. For the measurement of the moving direction angle, a gyroscope is installed on an axis perpendicular to the plane on which the axles of the vehicle are placed. For the measurement of the speed in the moving direction, a device for measuring an inclination angle of the road is required in addition to an accelerometer installed in the direction of axles.
The DR system measures the road inclination angle to obtain the velocity vector of the vehicle and enable the accelerometer to provide measurement data including a gravitational acceleration which can be calculated from the road inclination angle. The gravitational acceleration acts always in a direction normal to the surface of the earth spheroid (a spherical surface perpendicular to directions of the gravitational acceleration). When the direction of an axis of the accelerometer mounted in the vehicle (fixed in a particular direction within the vehicle) is changed due to a change in the road inclination angle, the gravitational acceleration component included in the output from the accelerometer is also changed. Accordingly, a pure motional acceleration of the vehicle can be obtained only when the gravitational acceleration component changing according to the road inclination angle is eliminated from the output of the accelerometer. However, neither the motional acceleration in the vehicle travel direction nor the gravitational acceleration component can be obtained without any information about the road inclination angle. Therefore, it is not possible to obtain the accurate speed of the vehicle.
FIG. 1 is a view explaining gravity compensation for an output from the accelerometer. Referring to FIG. 1, the road inclination angle refers to degrees from the horizontal plane which is perpendicular to the direction ±g of the gravitational acceleration. In FIG. 1, the road inclination angle θ is an angle made by the horizontal plane 10 perpendicular to the direction ±g of the gravitational acceleration and a sloped plane 20 extending in the moving direction of a car 30.
When the car 30 travels on the sloped plane 20 which rises at a predetermined angle θ from the horizontal plane 10 perpendicular to the direction ±g of the gravitational acceleration, the acceleration {right arrow over (a)} measured by the accelerometer mounted in the car 30 can be denoted by Equation 1.{right arrow over (a)}={overscore (r)}{right arrow over (a)}+{right arrow over (g)}  (1)
The acceleration {right arrow over (a)} measured by the accelerometer mounted in the car 30 includes the real acceleration {overscore (r)}{right arrow over (a)} and a component of the gravitational acceleration {right arrow over (g)} of the earth. The component of the gravitational acceleration {right arrow over (g)} is measured together with a rate of change in the actual speed of motion and may cause a big error in the measurement of speed.
Therefore, DR systems for vehicles are required to subtract the gravitational acceleration {right arrow over (g)} from the acceleration {right arrow over (a)} measured by the accelerometer in order to accurately measure the speed of a vehicle, and obtain the road inclination angle in order to measure the gravitational acceleration {right arrow over (g)}. To this end, a gyroscope or a clinometer is additionally provided in general DR systems.
In a conventional DR system, two or more gyroscopes are generally used to measure an angle of road inclination. In other words, a conventional DR system requires one gyroscope for determining the direction of motion and the other for determining the road inclination angle. Since gyroscopes are basically sensors for detecting a rate of change, a vehicle DR system integrates the output from the gyroscopes to obtain the road inclination angle. Therefore, when an angle of road inclination is measured using the gyroscopes, an error component can be integrated during integration of the output from the gyroscopes, thereby causing accumulation of estimated errors in the road inclination angle with the lapse of time.
Because of this drawback, a gyroscope is not used alone to measure the road inclination angle, but with the aid of an auxiliary sensor having no error accumulation property. Although an auxiliary sensor, such as a clinometer, can be additionally used to obtain an accurate angle of road inclination, the accelerometer already mounted in a vehicle is commonly used as an auxiliary sensor to reduce the number of sensors.
FIG. 2 is a diagram explaining a process of measuring a gravity component from an output of the accelerometer in the prior art. Referring to FIG. 2, a conventional DR system measures a relatively low-frequency component of the gravitational acceleration by passing an output from the accelerometer, which includes a gravitational acceleration component (a) and a real acceleration component (b), through a low pass filter (LPF) 40, and thereby calculates an angle of road inclination.
Although the conventional system has no error accumulation property, it is sensitive to the capacity of the accelerometer and insensitive to the change in the inclination angle. Also, since a low cut-off frequency is used to separate the gravity component, a time delay problem may be caused.
As a solution to these problems, an estimate obtained from a gyroscope, which is sensitive to instantaneous changes, is fused with that obtained from the accelerometer, which has no error accumulation property, to obtain an estimate of the road inclination angle which has less error accumulation and is insensitive to changes.
Since it is not possible to obtain an accurate gravitational acceleration using the accelerometer only or to calculate an accurate angle of road inclination from the obtained gravity acceleration, conventional vehicle DR systems utilize an additional gyroscope for estimating the road inclination angle. However, DR systems with an expensive gyroscope cannot be supplied at a low price.